Carbon and Nitrogen stable isotopes comparison in urban ecosystems along a Mediterranean latitudinal transect

In the last few decades, the Mediterranean area has been extensively impacted by prolonged and intense droughts and heat waves. These events, combined with heavily landscape management, ongoing for thousands of years in the region, have been affecting the ecosystems’ resources such as water availability and nutrient cycles. Mediterranean vegetation responds differently to natural and human-induced changes and within the same species, different compartments (e.g. leaves, branches and roots) may exhibit diverse responses to these stressors, providing valuable bioindicators. Stable isotopes, particularly carbon (C) and nitrogen (N), have become widely used as effective tools to study plant responses to environmental gradients such as plant water-use efficiency, nitrogen-use strategies and ecosystem functioning.

Within ITINERIS (Italian integrated environmental research infrastructures system) project, we present preliminary findings comparing d¹³C and d¹⁵N values in soil and vegetation samples from coastal urban and peri-urban parks along a latitudinal gradient in Italy. To evaluate the relationships between isotopic signatures and environmental changes, soil and vegetation samples were collected in three ICOS (Integrated Carbon Observation System) stations located along the coastal latitudinal transect (Pisa, Rome and Naples). Holm oak (Quercus ilex) was selected as a potential bioindicator of environmental changes and anthropogenic disturbances due to its abundant presence at all three sites. Soil and plant compartments (e.g., leaves of different ages, branches, pollen, fine roots) were collected and analysed at the start and end of the growing season to investigate the response of Mediterranean species down to organ level to a thermo-pluviometric gradient.

The main findings of this study have highlighted differences in C and N concentrations and isotope compositions across the latitudinal gradient and seasons. Capodimonte (Naples), the most southern site, reveals an enrichment in d¹³C and d¹⁵N in leaves compatible not only with a response to hot and dry climate, but also to a much higher degree of anthropization. Castel Porziano (Rome) exhibits a similar trend but lower d¹⁵N and N concentration. San Rossore (Pisa), the northernmost site is subject to less water stress during summer season, resulting in more diluted d¹³C and d¹⁵N values. The leaves in the fall showed higher d¹³C and N concentration compared to spring. Branches were about 1 ‰ enriched in ¹³C compared to leaves due to post-photosynthetic isotope fractionations. Further analyses are underway on other plant compounds, including pollen, to identify the most suitable bioindicators of Mediterranean species’ response to climate change and human impact, with potential applications in the region’s management and conservation strategies.